Most conventional motor vehicles are equipped with a brake system for selectively slowing or stopping movement of the vehicle in a controlled manner. Modern automotive braking systems may be grouped into two basic categories, disc brakes and drum brakes. A typical brake system may include a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. Regardless of type, brake assemblies are normally actuated by hydraulic, pneumatic, or mechanical pressure generated by an operator of the vehicle by, for example, depressing a foot pedal, pulling a hand lever, and like methods.
The primary components of a conventional disc brake assembly are the rotor or disc, caliper, one or more pistons, and two or more brake pads (also known as brake linings). The brake pads have a frictional lining supported by a rigid backing plate. The rotor is typically mounted so as to rotate with the axle of the wheel to be braked, configured to transmit braking forces from the caliper assembly to the wheel.
The caliper assembly, which houses the friction pads and pistons, is either solidly attached (fixed-type) or slidably mounted (floating-type) to a stationary, non-rotatable component of the vehicle, such as the vehicle frame. The piston(s), traditionally having a circular transverse cross-section, is slidably received in a cylinder bore of the caliper such that the centerline of the piston is parallel to the axis of rotation of the rotor. Through the application of pressure, whether it be hydraulic, mechanical, or pneumatic, the piston is forced to translate and, in so doing, to urge the brake pad(s) against a surface of the disc rotor, to thereby oppose and restrain rotational movement of the disc rotor through mechanical friction.
During a braking application, the brake pad and rotor may vibrate and generate a high-pitched brake noise, referred to in the art as “brake squeal”. For example, when the friction material of the brake pad contacts the rotor or drum of a brake assembly unevenly, the coupling causes the pad and rotor/drum to oscillate and vibrate (known as “force-coupled excitation”.) Additionally, as the brake assembly components heat up, the rotor may develop hot spots. The hot spots can cause the rotor to have regions of varying depth and friction, producing inconsistent levels of brake torque, and potentially exacerbating the aforementioned brake squeal.
Brake squeal is generally unappealing and unpleasant to vehicle occupants, pedestrians, etc., especially as vehicles are designed to be more comfortable and quieter. Hence, vehicle noise, vibration and harshness (NVH) is an important priority for today's original equipment vehicle manufacturers (OEM).
Efforts have been made to remedy or at least alleviate brake squeal. Some simple techniques like beveling or chamfering the linings, greasing the contact portion between the caliper and the linings, increasing the density of the backing plate, etc. help reduce squeal. Another approach to reducing or preventing “brake squeal” is to use a brake pad shim, which is interposed between the piston and backing plate of the brake pad in order to attenuate or reduce the magnitude of vibrations (resonance instability) of the brake pad and the disc rotor.
Brake shims control noise in three primary ways. First, they prevent and reduce the transmission and amplitude of vibrational forces that cause excitation of the caliper, pad assembly, and attached structure. This is traditionally accomplished by a viscoelastic damping material within the layering construction of the shims and/or through the bonding of the shim to the pad assembly. Second, shims add mass, even if minimally, to the brake linings which, in turn, dampens vibrations and oscillations by reducing reaction forces transmitted back into the brake piston using elastomer interface coatings on their surface. Finally, the brake shim can act as a thermal barrier to ensure consistent temperatures across the entire face of the pad, minimizing rotor hot spots and uneven lining wear, and providing a more uniform brake torque.